The present invention relates to alloys in which the essential constituent is aluminum, substrates coated with these alloys and the applications of these alloys, for example for forming thermal protection elements.
Diverse metals or metal alloys, for example aluminum alloys, have found numerous applications to date because of their valuable properties and in particular their mechanical properties, their good thermal conductivity, their lightness and their low cost. Thus, for example, cooking implements and equipment, anti-friction bearings, equipment mountings or supports and diverse articles obtained by molding are known.
However, the majority of these metals or metal alloys have drawbacks for some applications, associated with their inadequate hardness and resistance to wear and with their low resistance to corrosion, in particular in an alkaline medium.
Various attempts have been made to obtain improved aluminum alloys. Thus, European Patent 100287 describes a family of amorphous or microcrystalline alloys having improved hardness which can be used as reinforcing elements for other materials or in order to produce surface coatings improving the resistance to corrosion or wear. However, a large number of the alloys described in this patent are not stable at temperatures higher than 200.degree. C. and during a heat treatment, in particular the treatment to which they are subjected in the course of deposition on a substrate, they change structure: return to the microcrystalline state if the alloys concerned are essentially amorphous, coarsening of the grains in the case of the essentially microcrystalline alloys which initially have a particle size of less than 1 micron. This change in crystalline or morphological structure gives rise to a change in the physical characteristics of the material, which essentially affects its density. This results in the appearance of microcracks, causing fragility, which have an adverse effect on the mechanical stability of the materials.
Another family of alloys has been described in EP 356287. These alloys have improved properties. However, their copper content is relatively high.
Thermal stability is an indispensable property if an alloy is to be able to be used as a thermal barrier.
Thermal barriers are assemblies of one or more materials intended to restrict the heat transfer towards or from equipment parts and components in numerous domestic or industrial devices. For example, mention may be made of the use of thermal barriers in heating or cooking devices, irons at the attachment of the hot part to the casing and the thermal insulation; in cars, at several points, such as the turbocompressor, the exhaust silencer, insulation of the body, etc.; and in aeronautics, for example on the rear part of compressors and reactors.
Thermal barriers are sometimes used on their own in the form of a shield, but very often they are directly combined with the source of heat or with the part to be protected, for reasons of mechanical strength. Thus, use is made of mica sheets, ceramic sheets and the like in domestic household appliances, fitting them by screwing or sticking, or of sheets of agglomerated glass wool supported by a metal sheet. A particularly advantageous process for combining a thermal barrier with a part, in particular a metal part, consists in depositing the material constituting the barrier on a substrate in the form of a layer of predetermined thickness by a thermal spraying technique, such as plasma spraying for example.
Very often it is recommended to combine the thermal barrier with other materials also deposited in the form of a layer by thermal spraying. These other materials may be intended to ensure that the barrier is protected from external attack, such as, for example, mechanical shocks, a corrosive medium, and the like, or may serve as a sublayer for bonding to the substrate.
The material most frequently used in aeronautics to form thermal barriers is yttrium-containing zirconia, which withstands very high temperatures. The zirconia deposit is produced by plasma spraying using a conventional technique, using the powdered material as starting material. Zirconia has a low thermal diffusivity (.alpha.=10.sup.-6 m.sup.2 /s). However, it has a relatively high specific mass d, which is a drawback for some applications; moreover, some of its mechanical properties, such as the hardness and the resistance to wear and to abrasion are poor.
Other materials are used as a thermal barrier. Mention may be made of alumina, which has a specific mass lower than that of zirconia and a diffusivity and a specific heat higher than those of zirconia, but has unsatisfactory mechanical properties. Mention may also be made of stainless steels and some refractory steels which offer thermal insulation properties, but which have a high specific mass.